Dynamics

Geographically trapped meander positions

The mean path of the ARC as observed by MODAS-SSH is depicted below. Three crests and troughs are evident in the three year average. In agreement with the hydrographic study by Belkin and Gordon (1996) and the location of ARC RAFOS float trajectory segments, a first trough (T₁) is observed at 26.8°E, flanking the Agulhas Plateau to the north. Subsequent locations of three more crests and two more troughs are given in the following Table:

Feature	Trough 1	Crest 1	Trough 2	Crest 2	Trough 3	Crest 3
Longitude	26.8°E	29.7°E	32.6°E	35.5°E	38.9°E	42.9°E

fig_2.gif (25014 bytes)

Three year (1997-1999) average of the absolute sea-surface steric height anomaly from the MODAS. The 1.5m SSH isoline is indicated and located within the yellow transition from the red regime (positive anomalies near 2 m) to the blue regime ('negative' anomalies around 1 m elevation). Selected RAFOS float trajectory segments (white lines with dots) in the ARC region are superposed. Isobaths at 0, 1000 and 3000m are given (red lines), and areas above sea-level are shaded gray, areas shallower than 1000m are dark blue. All trajectory segments start west of the Agulhas Plateau.

The meandering path of the ARC can be substantiated through a spatially resolved histogram of high velocity events of the MODAS SSH field. The high velocity data, which obviously outlines the path of the ARC follows a zonal axis with slight tendency to south: 38°S at 25°E to 40°S at 45°E. Troughs and crests are found at the same position as in the averaged SSH field. A direct reflection of the increased meridional velocities is the higher frequency of velocities > 0.8 ms^-1 located along the connecting segments between troughs and crests in, e.g. directly east of the Agulhas Plateau.

fps_arc_08.gif (20355 bytes)

Histogram of velocities > 0.8 ms^-1. Thin red lines indicate bathymetry (0, 1000 and 3000m).

Westward propagation of cold core rings

The temporal evolution of the ARC is studied in a space-time diagram of MODAS SSH at a latitude of 37.5°S, a typical latitude for westward moving cyclones slightly to the north of the current's mean location. The Hovmöller diagram confirms the quasi-stationary meander position (vertical stripes east of 25°E) and, additionally, the occasional westward propagation of cyclonic signals north of the current.

hofmueller_375_v3.gif (35976 bytes)

Longitude-time (Hovmöller) diagrams for MODAS SSH at latitudes 37.5°S. Positive anomalies are red, negative anomalies are blue. Black arrows emphasize the propagation of cold core ring drifts.

An animation of this data confirms that the phase propagating patterns are linked to the shedding of eddies rather than the phase propagation of the entire meanders. During the three years of observations, a remarkable pattern of westward cold eddy migration was observed north of the ARC. During the transition from austral summer to fall (around April/May) eddies were shed from both trough T2 and T3. Eddies shed from T2 traveled west and merged with T1 around September. Eddies shed from T3 traveled synchronously to T2, with which they merged only to detach again and continue to T1, which they typically reached during the height of the following austral summer.

Animation of RAFOS float trajectories over MODAS SSH. Trajectories segments cover a 7 day period. Click on image to start the download (70MB ) of the animation, but patience in needed ! This is your chance for a coffee break! Tests to access the image acoss the Atlantic took 25 minutes at a data rate of about 50 KB/s. Once downloaded click the image again to start the animation. Trajectories will be visible only after March 1997, when the first floats were launched. Press your browsers Back button to return to this page.

Alternatively, the movie can be downloaded via anonymous ftp from po.gso.uri.edu/pub/downloads/oboebel/arc.zip (29MB only) and viewed with QuickTime, RealPlayer or similar programs. QuickTime offers the possibility to scroll back and forth and to stop the movie at any frame.